According to the known technique such drawbacks have been countered e.g. by providing the probe with a special internal pipe or channel system through which a coolant, e.g. water, is circulated, and by letting the probe operate intermittently, i.e. by discontinuing the gas sampling at intervals in order to clean the probe interior, e.g. by blowing it out with compressed air. However, from a production and operation point of view a cooling system of the kind described results in a more complex and hence more vulnerable probe construction, and the requirement for cleaning means that the gas stream cannot be monitored unintermittently.
Examples of this prior art technique are disclosed in the following patent Nos. DE-A-1813877, DE-A-1815045, DE-A-3327180 and DK-B-162800.
Furthermore, German published patent application No. DE-A-2603948 discloses a gas sampling probe comprising a gas inlet section, a filter section and a gas outlet section and having suction and analysis units coupled thereto and which is not provided with a cooling device but works intermittently like the known probes referred to above because it is necessary to blow out the interior of the probe at intervals in order to remove undesired cloggings.
Lastly, German publication No. DE-B-2224955 and English patent application No. GB-A-2040042 discloses examples of electrically and mechanically operating devices for preventing dust cloggings in gas sampling probes. However, the device according to DE-B-2224955 does not establish an electrical field until after the dust-filled smoke gas has passed through the filter unit and therefore the field does not prevent dust cloggings in the filter proper, while the mechanical purification device according to GB-A-2040042 is a rotating rod arranged inside the probe pipe along the entire length thereof, and which thus permanently occupies a substantial part of the pipe section and restricts the gas passage through the pipe, and additionally the probe according to this latter citation is provided with a cooling system which is complex per se.